Peel the sweet potatoes and cut them in to large pieces. Steam the sweet potatoes until completely soft (this should take about 20 minutes), then leave them in a colander to drain for at least an hour.
Whisk together all the sauce ingredients until smooth, then set aside.
The sweet potatoes should be mixed with the soy sauce, flour, salt, sugar, green onion, and red chile, preferably by hand, until the mixture is smooth and even. The mixture should be sticky; if it is runny add a bit more flour.
Melt some butter in a non-stick frying pan. For each cake, use a tablespoon to lift some of the mix into the pan and flatten with the back of a spoon to create a not-too perfect disc that is roughly 2 inches in diameter and 3/8 inch thick. Fry the cakes on medium heat for about six minutes on each side, or until you get a nice brown crust. Place in between two sheets of paper towels to soak up the excess butter. Serve hot or warm, with the sauce on the side.